This invention relates to a testing machine and a method for mechanically testing material samples.
A known testing machine is described in the Handbuch der Werkstoffprufung (Handbook for material testing) published by E. Siebel and L. Ludwig, Springerverlag 1985, Vol. 1, pages 15-17 and 54-55 which determines the relationship between loading and deformation or tension and extension of a material sample. This means that a testing machine which acts as a source of displacement or deformation has to be used which effects a defined deformation as an independent parameter and also measures the thereby resulting forces as a dependent parameter. The testing procedure must not become unstable and go out of control when the resistance to deformation of the sample decreases, in particular when the resistance to deformation changes suddenly.
It is further known from the journal "Materialprufung" (Material Testing), year 12 (1970), Nr.1, pages 1-6 that to fulfil these requirements testing machines are used which have control devices for regulated deformation during testing. In addition it is known from the "Technische Mitteilungen der Krupp Forschungsberichte" (Technical Information from the Krupp Reasearch Reports), Volume 24, 1966, H. 3, pages 79-88, to introduce an elastic by-pass to the sample in order to increase the effective spring constant of the testing machine. With the regulated deformation testing machine the actuator of the testing machine is operated by applying closed-loop control, so that either the displacement of the actuator or the machine traverse or the extension monitored in the sample by an extension transducer is compared with the predetermined theoretical value and the actual force applied by the testing machine for the displacement or the extension is measured.
Due to this the regulated deformation testing machine acts only as an indirect or an imitative source of displacement and deformation, which also means that only an indirect presetting of deformation is possible. The testing machine with the sum of its tractilities including that of the actuator acts in principal as a colinear spring in relation to the sample and therefore acts as a direct mechanical force at least in the case of small paths of deformation.
This leads to the following disadvantages:
During small spontaneous variations in the tractility of the sample during testing as occurs when passing through the yield limit of a steel sample or when microcracking occurs in concrete samples, the sample is further deformed disadvantageously in the direction of loading as the testing machine springs back in elastic compensation. This occurs spontaneouosly and independently of the testing machine actuator and is therefore not controllable. The sudden occurence of such phenomena often sets off a number of similar microprocesses in the sample structure until an equilibrium between the spring loads of the sample and the testing machine is again reached. This means that the structure of the sample is damaged unintentionally with a scattered falsifying influence on the results of the test.
The known testing machines cannot in principal prevent the already described compensation processes between the sample and the testing machines from taking place due to the indirect predetermination of deformation. In particular, the control function of the known testing machine in itself produces faulty test results due to the following: The control circuit can only react to the deviations from the preset value after they have occured, i.e. only after such a sudden compensation process between sample and testing machine has taken place and can only then correct the deformational deviation with the actuator as adjustment.
This deformation adjustment carried out by the control device leads to sizeable errors in the test results because the force patterns during an increase or a decrease of deformation in the plastic region are quite different from one another. When passing through the yield limit region of a steel sample for example, the lower yield stress limit is grossly incorrect, as after the extension leap has occured and the test machine has sprung back in consequence, this extension value is then deducted by the actuator to correct the test extension, which leads to the force applied decreasing much more steeply and running parallel to the elastic curve of the sample unlike during extension when the force diagram is relatively flat. This means that the value for the lower yield stress limit is far too low.
By setting a constant actuation or traverse speed of the testing machine it is not possible to obtain a constant deformation speed of the sample since the tractility in the colinear series machine-sample changes with a decreasing resistance to deformation of the sample and with that their individual contributions to deformation also change. In the plastic region and subjected to an approximately constant applied force, the sample receives all of the actuator motion as deformation and is then additionally deformed by the springing back of the testing machine when the loading is decreased. In this way the deformational speed of the sample, as for example during a tensile test, increases in an uncontrolled fashion up until ultimate failure even though a constant actuator speed is applied.
By positioning the sample and the testing machine in series the forces acting on the testing machine change automatically with the test force applied, which in the case of never to be ruled out unsymmetrical installation joints and an unsymmetrical construction of the testing machine lead to unsymmetrical load-dependent deformations, i.e. to a slanting position of the sample. It also automatically leads to a total unloading and to the connected springing back of the testing machine which occurs suddenly, in particular when testing brittle materials. Because of this, with time, the connections and the joints of the testing machine loosen and show signs of wear which can lead to faults, such as load-dependent slanted positions, which give rise to test errors and bad reproducibility, superimposed bending moments and faults in the actuator system.
The known testing machines are complicated and expensive. One tried to minimise the described disadvantages of these machines by using the stiffest machine construction possible, fast and sensitively reacting actuation devices and complicated control devices. The disadvantages could nevertheless not be prevented fully due to their inherent nature.
The known non-adjustable, elastic force by-pass devices as described in DE-PS 35 22 453 C2 and in the "Technische Mitteilungen der Krupp Forschungsberichte", 1966, H3 still have the mutual disadvantage that a force can only be applied to them if a force is also applied to the sample and at the same time can only enable a small deformation of the sample to take place due to the alteration of the force applied to the testing machine in the region of elastic deformation of the springs positioned parallel to the sample. These embodiments do not possess the crucial characteristic of an adjustable source of movement to enable the predetermination of a defined deformation force independent of the force applied and are not able to eliminate the influence of the deformation of the testing machine as this influence is inherent to this type of testing machine.